1. Field of the Invention
The present invention relates to a spread spectrum signal receiver. More specifically, the present invention relates to a spread spectrum signal receiver in which the data spectrum is spread by a spread code and transmitted in broad band.
2. Description of the Background Art
Communication using a narrow band modulation system has been conventionally used in the field of data communication. Such a system is advantageous in that demodulation at the receiver can be carried out by relatively small circuitry. However, it has a disadvantage that performance degrades because of multipath fading in an environment having multiple paths such as in a room (office, factory or the like).
By contrast, in a spread spectrum communication system, the spectrum of data is spread by a spread code and the data is transmitted in a broad band. Therefore, it is resistant against such frequency selective fading, and therefore the aforementioned disadvantage of narrow band communication can be eliminated. In order to implement digital processing in such a system, an A/D converter is used so that the data is converted and processed in the form of digital signals.
A conventional example of the receiver will be described with reference to the block diagram of FIG. 1. A signal from an input terminal 1 is amplified by a variable gain amplifier 2, passed through an A/D converter 3 and input to a correlator 4. An output from correlator 4 is input to a synchronizing circuit 6, and correlation synchronization is established at the correlated timing. By using a correlation synchronization pulse indicating the correlated and synchronized timing, the correlated output at that time is compared with an optimally set threshold value by a correlated output comparator 5, the output from comparison is smoothed by a filter 7, and thereafter it is applied to variable gain amplifier 2 so that the gain is controlled. In this manner, the maximum point where correlated output matches (hereinafter referred to as correlation spike) is always kept constant.
In order to fully make use of the performance of the receiver, the desired signal level to be input to A/D converter 3 has an optimal value, and it is not the case that any signal level may be input. The optimal value is known as the optimal quantization interval and in accordance with this interval, the signal amplitude of the input to A/D converter 3 must be optimized by means of variable gain amplifier 2. However, it is difficult to control the amplitude of the signal input to A/D converter 3 to be optimal by variable gain amplifier 2. Therefore, instead, the correlation spike is controlled such that it is kept constant.
If optimal signal amplitude is established, the amplitude of the correlation spike is constant. In other words, by controlling the gain of the variable gain amplifier 2 such that the amplitude of correlation spike is kept constant, the input signal comes to have an optimal amplitude, that is, optimal quantization interval is obtained, and hence the receiver comes to have best performance.
The operation in a steady state period will be described with reference to FIGS. 2A to 2C. In correlated output comparator 5, correlation spike P shown in FIG. 2A is compared with a threshold value E which is set in advance. Amplitude of correlation spike P varies from time to time. Meanwhile, the threshold value E is the optimal amplitude of the correlation spike. By comparing these two, a difference signal is output. The difference signal is as shown in FIG. 2B. By filtering the difference signal by a filter 7, a waveform shown in FIG. 2C is obtained. The waveform is determined by a constant of filter 7.
The purpose of filter 7 is to average variation caused by noise. By controlling variable gain amplifier 2 using the thus obtained output, the gain of amplifier 2 is lowered when correlated spike P is larger than the threshold value E, and the gain of amplifier 2 is increased when correlation spike P is smaller than the threshold value E. Feedback control is thus realized, whereby the peak of the correlation spike P is kept always matching the set threshold value E.
However, at the start of communication, correlation is not established. Therefore, the correlation synchronization pulse from synchronizing circuit 6 is generated not at the correct position of the correlation spike P. Therefore, in that case, correlated output comparator 5 compares the output with the threshold value E regardless of whether it is a correlation spike or not, and using the resulting difference signal, the variable gain amplifier 2 is controlled.
Now, let us consider the operation of the synchronizing circuit 6. In synchronizing circuit 6, of an initial synchronizing circuit and a synchronization protecting circuit constituting the synchronizing circuit 6, first, the initial synchronizing circuit operates to establish synchronization. Thereafter, synchronization is maintained by the synchronization protecting circuit. In this manner, the correlation synchronization pulse point is controlled such that it always appears at the correlation spike P. The initial synchronizing circuit compares an input signal with a set threshold value (the threshold provided for the synchronizing circuit 6 and not shown in FIG. 1), and if the input signal is higher, it assumes that the input signal is the correlation spike P. By repeating this operation several times at the same timing, synchronization is established.
However, in the conventional example, before initial synchronization is established, amplification rate of the variable gain amplifier 2 is unknown, and hence the signal amplitude entering A/D converter 3 is also unknown. This means that the amplitude of correlation spike P is also unknown. This makes it difficult to set the threshold value of the initial synchronizing circuit. Accordingly, there is a method for initial synchronization in which variable gain amplifier 2 is subjected to automatic gain control (AGC) so that the output signal from variable gain amplifier 2 is kept constant.
FIG. 3 shows this structure. Basically, this structure is the same as FIG. 1, except that an AGC circuit 8 is provided at the output of variable gain amplifier 2 for detecting power from the output and for keeping constant the amplitude. AGC circuit 8 is equivalent to any AGC circuit used in general communication equipment. In the structure shown in FIG. 3, at the time of initial synchronization, a switch 9 is switched so that the amplitude of the output from variable gain amplifier 2 is kept constant by the output from AGC circuit 8, and after initial synchronization is established, the variable gain amplifier 2 is controlled referring to the correlated output as shown in FIG. 1. This switching is performed using a synchronization flag FLG output from synchronizing circuit 6, indicating whether or not initial synchronization is established.
However, the conventional circuit is disadvantageous in that it requires additional AGC circuit 8. Though AGC circuit 8 can be implemented by a capacitor or a diode for detection, an analog filter or the like, these elements are all analog devices having large circuit scale and not suitable for integration.